Trilobite
Trilobites are a well-known fossil group of extinct marine arthropods that form the class Trilobita. Trilobites form one of the earliest known groups of arthropods. The first appearance of trilobites in the fossil record defines the base of the Atdabanian stage of the Early Cambrian period (521 million years ago), and they flourished throughout the lower Paleozoic era before beginning a drawn-out decline to extinction when, during the Devonian, almost all trilobite orders, with the sole exception of Proetida, died out. Trilobites finally disappeared in the mass extinction at the end of the Permian about 250 million years ago. The trilobites were among the most successful of all early animals, roaming the oceans for over 270 million years. When trilobites first appeared in the fossil record they were already highly diverse and geographically dispersed. Because trilobites had wide diversity and an easily fossilized exoskeleton an extensive fossil record was left behind, with some 17,000 known species spanning Paleozoic time. The study of these fossils has facilitated important contributions to biostratigraphy, paleontology, evolutionary biology and plate tectonics. Trilobites are often placed within the arthropod subphylum Schizoramia within the superclass Arachnomorpha (equivalent to the Arachnata), although several alternative taxonomies are found in the literature. Trilobites had many life styles; some moved over the sea-bed as predators, scavengers or filter feeders and some swam, feeding on plankton. Most life styles expected of modern marine arthropods are seen in trilobites, with the possible exception of parasitism (where there are still scientific debates). Some trilobites (particularly the family Olenidae) are even thought to have evolved a symbiotic relationship with sulfur-eating bacteria from which they derived food. Fossil record The earliest trilobites known from the fossil record are fallotaspids (order Redlichiida, suborder Olenellina, superfamily Fallotaspidoidea) and bigotinids (order Ptychopariida, superfamily Ellipsocephaloidea) dated to some 540 to 520 million years ago. Contenders for the earliest trilobites include Profallotaspis jakutensis (Siberia), Fritzaspis sp. (western USA), Hupetina antiqua (Morocco) and Serrania gordaensis (Spain). All trilobites are thought to have originated in present day Siberia, with subsequent distribution and radiation from this location. Fallotaspids lack facial sutures (see below), that is to say fallotaspids are thought to pre-date facial sutures (as opposed to a group that secondarily lost facial sutures). Fallotaspids are strongly suggested to be the ancestral trilobite stock: absence of facial sutures; apparently un-calcified protaspid stages and fallotaspids underlying (pre-dating) or co-existing with all other trilobite occurrences. However, recent developments suggest the picture is more complicated, and likely to change as more information comes to light. Origins Early trilobites show all of the features of the trilobite group as a whole; there do not seem to be any transitional or ancestral forms showing or combining the features of trilobites with other groups (e.g. early arthropods). Morphological similarities between trilobites and early arthropod-like creatures such as Spriggina, Parvancorina, and other "trilobitomorphs" of the Ediacaran period of the Precambrian are ambiguous enough to make detailed analysis of their ancestry far from compelling. Morphological similarities between early trilobites and other Cambrian arthropods (e.g. the Burgess Shale fauna and the Maotianshan shales fauna) make analysis of ancestral relationships difficult. However, it is still reasonable to assume that the trilobites share a common ancestor with other arthropods prior to the Ediacaran-Cambrian boundary. Evidence suggests significant diversification had already occurred prior to the preservation of trilobites in the fossil record, easily allowing for the "sudden" appearance of diverse trilobite groups with complex, derived characteristics (e.g. eyes). Divergence and extinction For such a long lasting group of animals, it is no surprise that trilobite evolutionary history is marked by a number of extinction events where unsuccessful groups perished while surviving groups diversified to fill ecological niches with more successful adaptations. Generally, trilobites maintained high diversity levels throughout the Cambrian and Ordovician periods before entering a drawn out decline in the Devonian culminating in final extinction of the last few survivors at the end of the Permian period. Evolutionary trends Principal evolutionary trends from primitive morphologies (e.g. eoredlichids) include the origin of new types of eyes, improvement of enrollment and articulation mechanisms, increased size of pygidium (micropygy to isopygy) and development of extreme spinosity in certain groups. Changes also included narrowing of the thorax and increasing or decreasing numbers of thoracic segments. Specific changes to the cephalon are also noted; variable glabella size and shape, position of eyes and facial sutures & hypostome specialization. Several morphologies appeared independently within different major taxa (e.g. eye reduction or miniaturization). Effacement is also a common evolutionary trend. It is the loss of surface detail in the cephalon, pygidium, or the thoracic furrows. The most notable examples of which happen in the orders Agnostida, Asaphida, and the suborder Illaenina of Corynexochida. It is believed that effacement is an indication of either a burrowing lifestyle or a pelagic one. Effacement poses a problem for taxonomists since the loss of details (particularly of the Glabella) can make the determination of phylogenetic relationships difficult. Pre-Cambrian Phylogenetic biogeographic analysis of Early Cambrian Olenellidae and Redlichidae suggests that a uniform trilobite fauna existed over Laurentia, Gondwana and Siberia before the tectonic breakup of the super-continent Pannotia between 600 million years ago and 550 million years ago. Tectonic break up of Pannotia then allowed for the diversification and radiation expressed later in the Cambrian as the distinctive olenellid province (Laurentia, Siberia and Baltica) and the separate Redlichid province (Australia, Antarctica and China). Break up of Pannotia significantly pre-dates the first appearance of trilobites in the fossil record, supporting a long and cryptic development of trilobites extending perhaps as far back as 700 million years ago or possibly further. Cambrian Very shortly after trilobite fossils appeared in the lower Cambrian, they rapidly diversified into the major orders that typified the Cambrian—Redlichiida, Ptychopariida, Agnostida and Corynexochida. The first major crisis in the trilobite fossil record occurred in the Middle Cambrian, surviving orders developed isopygus or macropygius bodies and developed thicker cuticles, allowing better defense against predators (see Thorax above). The end Cambrian mass extinction event marked a major change in trilobite fauna; almost all Redlichiida (including the Olenelloidea) and most Late Cambrian stocks went extinct. A continuing decrease in Laurentian continental shelf area is recorded at the same time as the extinctions, suggesting major environmental upheaval. Ordovician The Early Ordovician is marked by vigorous radiations of articulate brachiopods, bryozoans, bivalves, echinoderms, and graptolites with many groups appearing in the fossil record for the first time. Although intra-species trilobite diversity seems to have peaked during the Cambrian, trilobites were still active participants in the Ordovician radiation event with a new fauna taking over from the old Cambrian one. Phacopida and Trinucleioidea are characteristic forms, highly differentiated and diverse, most with uncertain ancestors. The Phacopida and other "new" clades almost certainly had Cambrian forebears, but the fact that they have avoided detection is a strong indication that novel morphologies were developing very rapidly. Changes within the trilobite fauna during the Ordovician foreshadowed the mass extinction at the end of the Ordovician allowing many families to continue into the Silurian with little disturbance. Ordovician trilobites were successful at exploiting new environments, notably reefs. However, the end Ordovician mass extinction did not leave the trilobites unscathed; some distinctive and previously successful forms such as the Trinucleioidea and Agnostida became extinct. The Ordovician marks the last great diversification period amongst the trilobites, very few entirely new patterns of organisation arose post-Ordovician; later evolution in trilobites was largely a matter of variations upon the Ordovician themes. By the Ordovician mass extinction vigorous trilobite radiation has stopped and gradual decline beckons. Silurian and Devonian Most Early Silurian families constitute a subgroup of the Late Ordovocian fauna. Few, if any, of the dominant Early Ordovician fauna survived to the end of the Ordovician, yet 74% of the dominant Late Ordovician trilobite fauna survived the Ordovician. Late Ordovician survivors account for all post-Ordovician trilobite groups except the Harpetida. Silurian and Devonian trilobite assemblages are superficially similar to Ordovician assemblages, dominated by Lichida and Phacopida (including the well-known Calymenina). However, a number of characteristic forms do not extend far into the Devonian and almost all the remainder were wiped out by a series of drastic Middle and Late Devonian extinctions. Three orders and all but five families were exterminated by the combination of sea level changes and a break in the redox equilibrium (a meteorite impact has also been suggested as a cause). Only a single order, the Proetida, survived into the Carboniferous. Carboniferous and Permian The Proetida survived for millions of years, continued through the Carboniferous period and lasted until the end of the Permian (where the vast majority of species on Earth were wiped out). It is unknown why order Proetida alone survived the Devonian. The Proetida maintained relatively diverse faunas in deep water and shallow water, shelf environments throughout the Carboniferous. For many millions of years the Proetida existed untroubled in their ecological niche. An analogy would be today's crinoids, which mostly exist as deep water species; in the Paleozoic era, vast 'forests' of crinoids lived in shallow near-shore environments. Final Extinction Exactly why the trilobites became extinct is not clear; with repeated extinction events (often followed by apparent recovery) throughout the trilobite fossil record, a combination of causes is likely. After the extinction event at the end of the Devonian period, what trilobite diversity remained was bottlenecked into the order Proetida. Decreasing diversity of genera limited to shallow water, shelf habitats coupled with a drastic lowering of sea level (regression) meant that the final decline of trilobites happened shortly before the end Permian mass extinction event. With so many marine species involved in the Permian extinction, the end of nearly 300 million successful years for the trilobite is hardly surprising. The closest extant relatives of trilobites may be the horseshoe crabs, or the cephalocarids. Fossil distribution Trilobites appear to have been exclusively marine organisms, since the fossilized remains of trilobites are always found in rocks containing fossils of other salt-water animals such as brachiopods, crinoids, and corals. Within the marine paleoenvironment, trilobites were found in a broad range from extremely shallow water to very deep water. Trilobites, like brachiopods, crinoids, and corals, are found on all modern continents, and occupied every ancient ocean from which Paleozoic fossils have been collected. The remnants of trilobites can range from the preserved body to pieces of the exoskeleton, which it sheds in the process known as ecdysis. In addition, the tracks left behind by trilobites living on the sea floor are often preserved as trace fossils. There are three main forms of trace fossils associated with trilobites: Rusophycus; Cruziana & Diplichnites – such trace fossils represent the preserved life activity of trilobites active upon the sea floor. Rusophycus, the resting trace, are trilobite excavations involving little or no forward movement and ethological interpretations suggest resting, protection and hunting. Cruziana, the feeding trace, are furrows through the sediment, which are believed to represent the movement of trilobites while deposit feeding. Many of the Diplichnites fossils are believed to be traces made by trilobites walking on the sediment surface. However, care must be taken as similar trace fossils are recorded in freshwater and post Paleozoic deposits, representing non-trilobite origins. Trilobite fossils are found worldwide, with many thousands of known species. Because they appeared quickly in geological time, and moulted like other arthropods, trilobites serve as excellent index fossils, enabling geologists to date the age of the rocks in which they are found. They were among the first fossils to attract widespread attention, and new species are being discovered every year. A famous location for trilobite fossils in the United Kingdom is Wren's Nest, Dudley in the West Midlands, where Calymene blumenbachi is found in the Silurian Wenlock Group. This trilobite is featured on the town's coat of arms and was named the Dudley Bug or Dudley Locust by quarrymen who once worked the now abandoned limestone quarries. Llandrindod Wells, Powys, Wales, is another famous trilobite location. The well-known Elrathia kingi trilobite is found in abundance in the Cambrian age Wheeler Shale of Utah. Spectacularly preserved trilobite fossils, often showing soft body parts (legs, gills, antennae, etc.) have been found in British Columbia, Canada (the Cambrian Burgess Shale and similar localities); New York State, U.S.A. (Ordovician Walcott-Rust quarry, near Russia, and Beecher's Trilobite Bed, near Rome); China (Lower Cambrian Maotianshan Shales near Chengjiang); Germany (the Devonian Hunsrück Slates near Bundenbach) and, much more rarely, in trilobite-bearing strata in Utah (Wheeler Shale and other formations), Ontario, and Manuels River, Newfoundland and Labrador. The French palaeontologist Joachim Barrande (1799–1883) carried out his landmark study of trilobites in the Cambrian, Ordovician and Silurian of Bohemia, publishing the first volume of Système silurien du centre de la Bohême in 1852.